Communications systems play an important role in modern society. Cable TV systems, for example, serve a large portion of American homes providing both entertainment and informational programming. A cable TV system typically includes a network of communications cables, such as coaxial cables. The coaxial cable includes a central conductor and an outer conductor with a dielectric material between the two conductors. A typical outside plant installation of coaxial cable, such as used for cable TV, may extend for many miles along a series of wooden utility poles.
Design of a communications cable route requires consideration of future demand for additional capacity along the route. This is particularly true for cable TV applications. When a cable TV coaxial cable is first installed, it must have sufficient communications capacity to meet expected growth along the route. Often, however, the growth may be underestimated. Then, an additional cable must be added along the existing cable route or a new cable substituted for the existing cable. In either case, a cable must be physically placed along the route by traditional labor-intensive outside plant cable installation methods. The labor to install the new cable may often be a significant component of the total project cost and may exceed the cost of purchasing the new cable. Moreover, in a typical outside plant environment, the addition of a cable may create right-of-way problems, such as requiring higher leasing fees or causing clearance problems with existing power or other telecommunication cables.
As fiber optic cable and associated electronics become more cost competitive with coaxial cable systems for cable TV, fiber optics offer an attractive advantage in terms of upgrading the capacity, for example, of an existing coaxial cable route. A fiber optic cable typically has an inherently large bandwidth that can be incrementally utilized by merely upgrading terminal electronics. In many cases, the upgrading of the electronics may simply require the addition of modular units to the existing equipment. In addition, fiber optic cables may be less susceptible to interference than coaxial cables, and fiber optic cables may allow longer distances between repeaters, or signal amplifiers than coaxial cable systems.
Coaxial cable systems, including associated electronics, are still oftentimes very cost effective for cable TV companies. In addition, cable TV companies may have a substantial investment in electronics and other components for their existing coaxial cable systems. Thus, a wholesale conversion to fiber optic cable and electronics may not be cost justified. In addition, it may be highly speculative to replace a coaxial cable route with fiber optics when the future demand for increased capacity is uncertain. What is needed is a technology that provides for greater flexibility on the part of cable TV companies to choose coaxial or fiber optic systems or a combination of both, yet which is cost effective.
One possible approach to providing for future growth of a coaxial cable route is suggested by the prior art wherein a hybrid coaxial and fiber optic cable is disclosed. For example, U.S. Pat. No. 4,695,127 to Ohlhaber et al. discloses a hybrid cable wherein an optical fiber is positioned in a channel located between the center conductor of a coaxial cable and the outer conductor during the manufacture of the cable. The cable is directed to providing secure communications transmission wherein the cable has the outward appearance of a coaxial cable, yet which has an inner optical fiber for carrying confidential information. In one embodiment, the channel is formed by a spiral spacer. In another embodiment, several channels are provided by oversized buffer tubes, each containing an individual optical fiber. The buffer tube may be filled with a conventional water blocking compound.
Another type of hybrid coaxial and fiber optic cable is disclosed in U.S. Pat. No. 5,042,904 to Story et al. and assigned to the assignee of the present invention. The patent is directed to providing a talk path for maintenance activities along a cable route, such as to repair damage caused by a cable digin. Typically the cable may be a fiber optic cable and the talk path may be a twisted copper pair. In one embodiment, the talk path may be an optical fiber positioned in a longitudinally extending cavity in the outer jacket of a coaxial cable. The cable jacket includes a series of side-by-side longitudinally extending cavities. The cavities provide enhanced crush and cut-through resistance to the cable. However, the optical fiber is a conventional optical fiber that is installed during the extrusion of the cable jacket.
While a hybrid coaxial and fiber optic cable may provide the flexibility useful to cable TV companies, such hybrid cables are often relatively expensive. In addition, the designer must speculate on whether a route may eventually require a fiber optic cable in the future. If the hybrid cable is installed and not needed in the future, an unnecessary expense is incurred. If the hybrid cable is not installed, but later needed, an expensive additional or replacement cable is needed.
It is known that relatively small diameter fiber optic cables containing several optical fibers may be "blown" or propelled into an existing duct by passing a fluid through the duct and adjacent to the fiber optic cable. U.S. Pat. Nos. 4,930,860 and 4,976,519 to Tansey et al. and Davey et al., respectively, disclose a propellable optical fiber cable that may be placed in a duct by fluid drag produced by a flow of fluid, such as air, through the duct.
The propellable fiber optic cables typically include one or more optical fibers and an outer sheath having physical properties that allow it to be advanced through the duct by drag produced by a fluid flow, such as an air flow, adjacent the cable and through the duct. For example, the outer sheath may be foam which has a low density thereby reducing the weight and making the cable easier to propel by the fluid flow. The cable may have a predetermined stiffness and surface properties, such as texture and coefficient of friction, that also contribute to ease of propelling by a fluid flow.
The propellable fiber optic cable has been used in conjunction with a copper twisted pair cable for intra-building use, such as for a local area network (LAN). The twisted pair cable includes an empty duct or tube extending therealong beneath a common outer jacket. The user may install and use the twisted pair cable, and then, at a later time, add the propellable fiber optic cable through the tube.
Presently available conventional coaxial cables and installation techniques do not provide for a flexible approach to upgrading communications capacity along an existing coaxial cable route. Hybrid coaxial and fiber optic cables provide for future flexibility, but are relatively expensive, and the expense must be borne during the initial installation when the future capacity demand for the route is uncertain at best. Intra-building twisted pair cables and a later added propellable fiber optic cable for LAN interconnections do not address the problem of upgrading an existing coaxial cable route, especially since a coaxial cable, by the nature of its construction, must have a predetermined concentric relationship between the center conductor and surrounding outer conductor.